Perimeter fences are known having intruder alarms in the form of deflection sensors fixed to the fence posts and having the fence wires fixedly mounted thereto. Unauthorized tampering with the fence wires causes deformation of the deflection sensors attached thereto and permits such tampering to be detected and monitored. By such means it is possible not only to determine the act of attempted intrusion but also its location.
International Patent publication No. WO 9,309,521 in the name of Vindicator Corporation discloses a strain gauge deflection sensor having a flexible housing connected at one end to a support and at the other end to a taut wire of a perimeter fence. The interior of the housing contains a full bridge strain gauge having a circuit board carrying four strain gauge elements and an amplifier for the differential voltage obtained from the bridge circuit corresponding to flexure of the circuit board upon deflection of the taut wire. The output from the amplifier is fed to a data processing system providing an alarm signal.
Such deflection sensors are in common use but suffer from the drawback that they are subject to stray ambient electromagnetic radiation which can produce an intrusion signal even when no attempted intrusion is taking place. The only way to avoid the undesirable effect of such stray radiations has, so far, been to decrease the overall sensitivity of the strain gauge circuit but this solution militates against their use in very high security perimeter fences where extreme sensitivity is required.